Effective June 1, 2022 the cost of an Additional Platinum card on a Business Platinum account will increase to $350. This reinforces my expectation that the cost of additional Platinum cards on personal Platinum accounts will increase this year,
$350 is 50% of the card's annual fee. At that point, hypothetical P2s would be better off applying for a card themselves and churning the credits, personal or business.
$350 for 3 cards on personal might be acceptable, but still that's still very steep.